1. Field of the Invention
The present invention relates to a heat-resistant, light-shielding film, method for producing the film, and aperture and light intensity adjusting device using the film, more specifically a heat-resistant, light-shielding film having high light shielding capacity, high heat resistance, high sliding characteristics, low surface gloss and high electroconductivity, and useful for optical device parts, e.g., shutter or aperture blades for aperture or lens shutter for digital cameras and digital video cameras, and aperture blades for adjusting light intensity for projectors; method for producing the film; and aperture and light intensity adjusting device using the film.
2. Description of the Prior Art
Reentry, shutter blades or aperture blades for cameras have been required to be lighter and have higher sliding characteristics as shutter speed increases, because they are subjected to starting/stopping cycles in a very short time. At the same time, they are basically required to have light shielding capacity, because they cover photosensitive materials, e.g., films, or front surfaces of imaging devices, e.g., CCDs, to shield light. Moreover, they are required to have sufficient lubricity for smooth motion of blades for which they are used, because a plurality of blades work while overlapping each other in optical devices. Still more, they are required to have a low surface reflectivity to prevent leakage of light between blades. Still more, they are required to be resistant to heat, because cameras for which they are used may evolve high temperature inside depending on service environments.
On the other hand, light-shielding films for light intensity adjusting aperture blades for liquid-crystal projectors as projectors for presentation or image viewing devices for home theaters or the like are required to have characteristics similar to those for digital cameras and digital video cameras, or in particular higher heat resistance.
The commercial light-shielding films described above are generally supported by a base of plastic film, e.g., polyethylene terephthalate (PET) film, or thin metallic plate of stainless steel (SUS), SK, Al or the like. When a light-shielding film supported by a metallic base is used as a shutter or aperture blade in a camera, the metallic plates will graze with each other as the blades are opened and closed to create large noise. The blade in a liquid-crystal projector is required to move at a high speed to reduce brightness changes when images are changed, with the result that the blades will graze with each other to repeatedly create noise. To operate the blades at a lower speed to reduce noise will produce unstable images, because light intensity adjustment may not sufficiently follow changed images.
Recently, light-shielding films have been mainly supported by a plastic film, in order to solve the above problems and to reduce the weight. Moreover, they are required to be electroconductive to reduce evolution of dust. Therefore, it is accepted that they are required to satisfy the characteristics of high light shielding capacity, high heat resistance, low surface gloss, high sliding characteristics, high electroconductivity and low soot evolution. Various film materials and structures have been proposed to satisfy the characteristics which light-shielding films are required to have.
For example, Patent Document 1 discloses a light-shielding film with a resin film, e.g., polyethylene terephthalate (PET) film, impregnated with fine, black, electroconductive particles of carbon black, titanium black or the like to provide the film with light-shielding capacity and electroconductivity, and is matting treated at least on one side to decrease surface gloss, wherein the fine black particles work to absorb light emitted from a light source, e.g., lamp, for high light-shielding capacity, low surface gloss and high electroconductivity.
Patent Document 2 discloses a light-shielding film with a resin film coated with a thermosetting resin layer containing a black pigment, e.g., carbon black, having light-shielding capacity and electroconductivity, lubricant and delustering agent to have light-shielding capacity, electroconductivity, lubricity and low surface gloss.
Patent Document 3 discloses a light-shielding material comprising a metallic blade material of aluminum alloy or the like, coated with a hard carbon film.
Patent Document 4 discloses a light-shielding blade structure with a plastic base coated and reinforced with a prepreg sheet of thermosetting resin containing carbon fibers on each side, to improve blade rigidity.
Light-shielding films have been widely used as light-shielding blade materials for optical devices, e.g., digital cameras, digital video cameras and liquid-crystal projectors. Recently, liquid-crystal projectors have been required to produce high-quality, bright images of high contrast in bright atmospheres, e.g., those in living rooms. As a result, a lamp as a light source produces high output for brighter images, which tends to increase temperature in an aperture device for light intensity adjustment. Light-shielding films for light intensity adjustment, when irradiated with light of high output, are placed in atmospheres in which they tend to thermally deform.
Light-shielding films with a base of polyethylene terephthalate or the like have been widely used because of their low specific gravity. However, polyethylene terephthalate has a low thermal deformation temperature and low mechanical strength, e.g., modulus of elongation, with the result that light-shielding blades of polyethylene terephthalate may be deformed by vibrations and shocks evolving in the service or braking period as output of light sources.
Moreover, light-shielding films are matting-treated by sand blasting to have low surface gloss and high sliding characteristics. This treatment brings another effect of improving image visibility by scattering incident light to decrease surface gloss. This treatment is considered to prevent deterioration of sliding characteristics, because of controlled growth of contacting area between the light-shielding films even when they come into contact with each other.
Digital cameras, digital video cameras and liquid-crystal projectors inevitably include a plurality of light-shielding films as shutter or aperture blades which work while overlapping each other. Light-shielding films which include an organic light-shielding material, lubricant and delustering agent have been subjected to severer service conditions, e.g., temperature or humidity, in digital cameras, digital video cameras and liquid-crystal projectors. In liquid-crystal projectors, in particular, temperature inside of their devices (light-intensity adjusting devices, aperture devices and so forth) is increasing to near 200° C., as output of light sources, e.g., lamps to produce images of higher brightness, increases, as discussed above. In these severe environments, the conventional light-shielding films described above show unfavorable conditions, e.g., deformation or discoloration, and cause practical problems.
Light-shielding films, when used at high temperature of 200° C. or higher, will be greatly deformed by heat, even when they have a fine irregular structure on the surface, with the result that they can no longer work at a high speed when they come into contact with each other, and have deteriorated sliding characteristics and surface gloss because they are grazing with each other more frequently irregularly. As a result, the devices in which they are used, e.g., digital cameras, digital video cameras or liquid-crystal projectors, may no longer exhibit their intended functions.
Matting treatment of a plastic film as a base forms fine irregularities on the surface to bring effects of improving adhesion between the base and coating film with which the base is coated and reducing surface gloss. Sand blasting, however, provides film surface roughness which depends on material and particle size of a shot material and pressure at which it is ejected, among others. A shot material of large particle size may be removed from the surface by an adequate method, e.g., washing or flushing with water. A shot material of fine particles smaller than 1 μm, on the other hand, cannot be completely removed, and the particles remain on the surface in no small quantity, even it is washed. The remaining shot material may cause separation of the film, e.g., light-shielding film of metal alloy formed on the base film, and also separation of the residual shot material from the film at high temperature to which the light-shielding film is exposed, because of a thermal stress created by differential thermal expansion coefficient between the shot material and film. These troubles can have adverse effects on the surrounding parts, and a device in which the film is used may no longer exhibit its intended functions.
Patent Documents
[Patent Document 1]: JP-A-1-120503
[Patent Document 2]: JP-A-4-9802
[Patent Document 3]: JP-A-2-116837
[Patent Document 4]: JP-A-2000-75353